The present invention relates to an optical head provided with a plurality of light sources including a short-wavelength laser source (for emitting a beam with a wavelength between 350 nm and 500 nm), which is used for recording and/or reproduction of an information recording medium such as an optical disk. The present invention also relates to a recording and reproduction method of recording and/or reproduction of an information recording medium using a plurality of light sources including a short-wavelength laser source.
FIG. 13 is a structural view of a conventional optical head using a short-wavelength laser source. As a light source used for recording and reproduction of an information recording medium, a short-wavelength laser source (for emitting a beam with a wavelength of 425 nm) is used, which includes a semiconductor laser (for emitting a beam with a wavelength of 850 nm) and an SHG (Second Harmonic Generation) element for shortening the wavelength of an incident beam to half by generating second harmonics. As shown in the figure, a laser beam 103 emitted from a light source including a semiconductor laser 101 and an SHG element 102 for shortening the wavelength of the laser beam emitted from the semiconductor laser 101 to half is converted into parallel light by a collimate lens 104, which is reflected to an information recording medium 109 side by a mirror 106 for guiding light upward and is converted into circularly polarized light by a xc2xc wave plate 107. After that, the circularly polarized light is focused on an information recording surface 110 of the information recording medium 109 by an objective lens 108. Then, reflected light from the information recording surface 110 is converted into linearly polarized light whose polarization direction is perpendicular to a polarization direction of the laser beam 103 in an incoming path (a path from the light source to the information recording surface 110) by the xc2xc wave plate 107, which is then reflected by a polarization be am splitter 105. After that, the reflected light enters a detection lens 111 and is focused on receiving optics 112, thus detecting a signal. 
In a conventional optical head for recording and reproducing a high-density information recording medium using a short-wavelength laser source for emitting a beam with a short wavelength within a range between 350 nm and 500 nm, when a beam with a wavelength of red light and a beam with an infrared wavelength enter the same optical system including an objective lens, an aberration occurs due to chromatic dispersion caused by the material of the objective lens with respect to the beams with such wavelengths. Therefore, with respect to DVDs (with an operating wavelength of about 650 nm), DVD-Rs (with an operating wavelength of about 650 nm), CDs (with an operating wavelength of about 800 nm), CD-Rs (with an operating wavelength of about 800 nm) and CD-RWs (with an operating wavelength of about 800 nm) that use such wavelengths, information cannot be recorded and/or reproduced using the same optical system as that in the above-mentioned optical head, which has been a problem.
The present invention is intended to solve the above-mentioned problem and to provide an optical head that can record and/or reproduce information using the same optical system with respect to a plurality of information recording media with different specifications employing different operating wavelengths, such as high-density disks (with an operating wavelength of about 350 nm-500 nm), DVDs, DVD-Rs, CDs, CDRs, CD-RWs, and the like. Particularly, the present invention aims to provide a small optical head that is provided with a light source for emitting light with a wavelength between 350 nm and 500 nm and at least one of light sources for emitting light with a wavelength between 600 nm and 700 nm and for emitting light with a wavelength between 700 nm and 900 nm and enables excellent property of focusing the respective lights to be obtained using a common single optical system. The present invention also is intended to provide a recording and reproduction method in which, with respect to a plural kinds of information recording media with different operating wavelengths, information can be recorded and/or reproduced using an optical head provided with a plurality of light sources for emitting lights with different wavelengths and a common single optical system.
In order to achieve the aforementioned objects, the present invention employs the following configurations.
An optical head according to a first configuration of the present invention includes: a first light source for emitting light with a first wavelength; a second light source for emitting light with a second wavelength longer than the first wavelength; and an objective lens for focusing the light with the first wavelength and the light with the second wavelength on information recording surfaces of information recording media corresponding to the lights with the respective wavelengths. The objective lens is designed so as to reduce aberration with respect to the light with the first wavelength. An optical-path length from the second light source to the objective lens is shorter than that from the first light source to the objective lens so that aberration caused by the objective lens due to chromatic dispersion caused by a material of the objective lens is reduced with respect to the light with the second wavelength.
By setting both the optical-path lengths as described above, the aberration caused by the objective lens with respect to the light with the second wavelength can be reduced, thus improving the property of focusing the light with the second wavelength. Therefore, using the same optical system, recording and/or reproduction can be carried out with respect to an information recording medium corresponding to the second wavelength in addition to an information recording medium corresponding to the first wavelength. Furthermore, since a common optical system can be used, the optical head can be simplified in configuration and can be reduced in size.
An optical head according to a second configuration of the present invention includes: a first light source for emitting light with a first wavelength; a second light source for emitting light with a second wavelength longer than the first wavelength; and an objective lens for focusing the light with the first wavelength and the light with the second wavelength on information recording surfaces of information recording media corresponding to the lights with the respective wavelengths. The objective lens is designed so as to reduce aberration with respect to the light with the second wavelength. An optical-path length from the first light source to the objective lens is longer than that from the second light source to the objective lens so that aberration caused by the objective lens due to chromatic dispersion caused by a material of the objective lens is reduced with respect to the light with the first wavelength.
By setting both the optical-path lengths as described above, the aberration caused by the objective lens with respect to the light with the first wavelength can be reduced, thus improving the property of focusing the light with the first wavelength. Therefore, using the same optical system, recording and/or reproduction can be carried out with respect to an information recording medium corresponding to the first wavelength in addition to an information recording medium corresponding to the second wavelength. Furthermore, since a common optical system can be used, the optical head can be simplified in configuration and can be reduced in size.
An optical head according to a third configuration of the present invention includes: a first light source for emitting light with a first wavelength; a second light source for emitting light with a second wavelength longer than the first wavelength; a third light source for emitting light with a third wavelength longer than the second wavelength; and an objective lens for focusing the respective lights with the first, second, and third wavelengths on information recording surfaces of information recording media corresponding to the lights with the respective wavelengths. The objective lens is designed so as to reduce aberration with respect to the light with the first wavelength. Optical-path lengths from the respective second and third light sources to the objective lens are shorter than an optical-path length from the first light source to the objective lens so that the aberration caused by the objective lens due to chromatic dispersion caused by a material of the objective lens is reduced with respect to the respective lights with the second and third wavelengths.
By setting the respective optical-path lengths as described above, the aberration caused by the objective lens with respect to the lights with the second and third wavelengths can be reduced, thus improving the property of focusing lights with the second and third wavelengths. Therefore, using the same optical system, recording and/or reproduction can be carried out with respect to information recording media corresponding to the second and third wavelengths in addition to an information recording medium corresponding to the first wavelength. Furthermore, since a common optical system can be used, the optical head can be simplified in configuration and can be reduced in size.
An optical head according to a fourth configuration of the present invention includes: a first light source for emitting light with a first wavelength; a second light source for emitting light with a second wavelength longer than the first wavelength; a third light source for emitting light with a third wavelength longer than the second wavelength; and an objective lens for focusing the respective lights with the first, second, and third wavelengths on information recording surfaces of information recording media corresponding to the lights with the respective wavelengths. The objective lens is designed so as to reduce aberration with respect to the light with the second wavelength. Optical-path lengths from the respective second and third light sources to the objective lens are shorter than an optical-path length from the first light source to the objective lens so that the aberration caused by the objective lens due to chromatic dispersion caused by a material of the objective lens is reduced with respect to the respective lights with the first and third wavelengths.
By setting the respective optical-path lengths as described above, the aberration caused by the objective lens with respect to the lights with the first and third wavelengths can be reduced, thus improving the property of focusing lights with the first and third wavelengths. Therefore, using the same optical system, recording and/or reproduction can be carried out with respect to information recording media corresponding to the first and third wavelengths in addition to an information recording medium corresponding to the second wavelength. Furthermore, since a common optical system can be used, the optical head can be simplified in configuration and can be reduced in size.
In the first and second optical heads, it is preferable that an aberration correction member for further reducing the aberration caused by the objective lens due to the chromatic dispersion caused by the material of the objective lens is provided in the common optical path from the first and second light sources to an incident plane of the objective lens. According to this, the aberration caused by the objective lens with respect to the lights with the first and second wavelengths can be reduced by inserting the aberration correction member. Thus, the property of focusing the lights with the first and second wavelengths can be further improved.
In the third and fourth optical heads, it is preferable that an aberration correction member for further reducing the aberration caused by the objective lens due to the chromatic dispersion caused by the material of the objective lens is provided in the common optical path at least from the first and second light sources to an incident plane of the objective lens. According to this, the aberration caused by the objective lens at least with respect to the lights with the first and second wavelengths can be reduced by inserting the aberration correction member. Thus, the property of focusing the lights with the first, second, and third wavelengths can be further improved.
In the above, the aberration correction member may be a hologram element for correcting chromatic aberration caused by the objective lens with respect to the light with the first wavelength. According to this, the aberration due to the chromatic dispersion caused by the material of the objective lens with respect to the light with the first wavelength can be reduced, thus improving the focusing property of the objective lens. Further, by using the hologram element, the size of the optical head can be reduced.
Alternatively, in the above, the aberration correction member may be a hologram element for correcting chromatic aberration caused by the objective lens with respect to the light with the second wavelength. According to this, the aberration due to the chromatic dispersion caused by the material of the objective lens with respect to the light with the second wavelength can be reduced, thus improving the focusing property of the objective lens. Further, by using the hologram element, the size of the optical head can be reduced.
In the above, it is preferable that the hologram element is integrally formed on a surface of the objective lens. According to this, the optical system including the lens and the hologram element can be reduced in weight and size, thus reducing the load on an actuator for actuating the lens.
Further, in the first and second optical heads, the aberration correction member may be a collimate lens for converting divergent light from the first or second light source substantially into parallel light. According to this, the tolerance for positioning accuracy of the objective lens can be increased in a plane perpendicular to an optical axis of light that has been converted into parallel light by the collimate lens out of the divergent lights from the first and second light sources.
Similarly, in the third and fourth optical heads, the aberration correction member may be a collimate lens for converting divergent light from the first, second, or third light source substantially into parallel light. According to this, the tolerance for positioning accuracy of the objective lens can be increased in a plane perpendicular to an optical axis of light that has been converted into parallel light by the collimate lens out of the divergent lights from the first, second, and third light sources.
In this case, the light emitted from the third light source may be allowed to enter the objective lens directly without passing through the collimate lens. According to this, recording and/or reproduction of a plurality of optical disks with different specifications, such as, for example, high-density optical disks, DVDs, DVD-Rs, CDs, CD-Rs, or CD-RWs, can be carried out easily in the same optical system.
Further, the aberration correction member may be a collimate lens designed to cause aberration so as to reduce the aberration caused by the objective lens due to the chromatic dispersion caused by the material of the objective lens. In other words, the collimate lens is allowed to cause aberration intentionally so as to cancel or reduce the chromatic aberration caused by the objective lens. As a result, the aberration in the optical system as a whole can be further reduced.
In the third or fourth optical head, it is preferable that the optical-path length between the third light source and the objective lens is set so that the NA (numerical aperture) of the objective lens is 0.5 or less with respect to the light with the third wavelength. According to this, recording and/or reproduction can be carried out easily with respect to information recording media, such as, for example, CDs, CD-Rs, or CD-RWs.
In the above-mentioned first to fourth optical heads, it is preferable that an Abbe number of the material of the objective lens is different from that of a material of the aberration correction member so that the aberration caused by the objective lens due to the chromatic dispersion caused by the material of the objective lens is reduced. According to this, the chromatic aberration caused by the objective lens can be compensated by the aberration correction member.
In the above-mentioned first to fourth optical heads, it is preferable that the Abbe number of the material of the objective lens is at least 55. By using a material causing less chromatic dispersion as a material of the objective lens, the chromatic aberration caused by the objective lens can be reduced.
In the above-mentioned first to fourth optical heads, the objective lens may be a single aspherical lens designed so as to correct aberration with respect to light with any one of the wavelengths. According to this, wavefront aberration easily can be reduced simply by arranging the respective light sources so as to vary the optical-path lengths between the respective light sources and the objective lens so that aberration caused by the objective lens with respect to lights that are not subjected to the aberration correction in the objective lens can be reduced.
Alternatively, in the above-mentioned first to fourth optical heads, the objective lens may be a combination lens formed of spherical or aspherical lenses in which chromatic aberration and wavefront aberration are corrected with respect to the lights with the respective wavelengths. According to this, wavefront aberration easily can be reduced simply by arranging the respective light sources so as to vary the optical-path lengths between the respective light sources and the objective lens.
In the above-mentioned first or second optical head, it is preferable that the first and second light sources emit light selectively. According to this, recording and/or reproduction can be carried out with respect to information recording surfaces of different information recording media corresponding to two different wavelengths.
In the above-mentioned third or fourth optical head, it is preferable that the first, second, and third light sources emit light selectively. According to this, recording and/or reproduction can be carried out with respect to information recording surfaces of different information recording media corresponding to three different wavelengths.
In the above-mentioned first or second optical head, it is preferable that the first wavelength is in a range between 350 nm and 500 nm and the second wavelength is in a range between 600 nm and 700 nm. According to this, using the same optical head, recording and/or reproduction can be carried out with respect to a plurality of optical disks with different specifications, such as high-density disks, DVDs, DVD-Rs, or the like.
In the above-mentioned third or fourth optical head, it is preferable that the first wavelength is in a range between 350 nm and 500 nm, the second wavelength in a range between 600 nm and 700 nm, and the third wavelength in a range between 700 nm and 900 nm. According to this, using the same optical head, recording and/or reproduction can be carried out with respect to a plurality of optical disks with different specifications, such as high-density optical disks, DVDs, DVD-Rs, CDs, CD-Rs, CD-RWs, or the like.
A recording and reproduction method according to the present invention is a method in which, with respect to information recording media having information recording surfaces and protective layers thereon, information is recorded on and/or reproduced from the information recording surfaces by projecting light on the information recording media from the side of the protective layers. In the method, an optical head is used, which includes: a first light source for emitting light with a first wavelength; a second light source for emitting light with a second wavelength longer than the first wavelength; and an objective lens for focusing the light with the first wavelength and the light with the second wavelength on the information recording surfaces of information recording media corresponding to the lights with respective wavelengths. The objective lens used is designed so as to reduce aberration with respect to the light with the second wavelength. The thickness of the protective layer on the information recording surface corresponding to the light with the first wavelength is allowed to be different from that of the protective layer on the information recording surface corresponding to the light with the second wavelength so that aberration caused by the objective lens due to chromatic dispersion caused by a material of the objective lens is reduced with respect to the light with the first wavelength.
By varying the thickness of the protective layers in the information recording media depending on the wavelengths of lights emitted from the light sources used, chromatic aberration caused by the objective lens can be reduced. As a result, using the same optical system, recording and/or reproduction can be carried out with respect to a plurality of information recording media with different operating wavelengths. Since a common optical system can be used, the optical head can be simplified in configuration and can be reduced in size.
In the aforementioned recording and reproduction method, it is preferable that the lights with the first and second wavelengths entering the objective lens are substantially parallel lights, the protective layer on the information recording surface corresponding to the light with the second wavelength has a thickness in a range of 0.55 mm to 0.65 mm, and the protective layer on the information recording surface corresponding to the light with the first wavelength has a thickness in a range of 0.65 mm to 0.75 mm. According to this, recording and/or reproduction can be carried out with respect to the information recording surfaces of the different information recording media corresponding to the respective first and second wavelengths. In addition, by using the parallel lights, the tolerance for positioning accuracy of the objective lens can be increased in a plane perpendicular to an optical axis.
In the aforementioned recording and reproduction method, it is preferable that an Abbe number of the protective layer on the information recording surface corresponding to the light with the first wavelength is allowed to be different from that of the protective layer on the information recording surface corresponding to the light with the second wavelength so that the aberration caused by the objective lens due to the chromatic dispersion caused by the material of the objective lens is further reduced with respect to the first wavelength. By varying the Abbe numbers of the protective layers in the information recording media depending on the wavelengths of lights emitted from the light sources used, the chromatic aberration caused by the objective lens can be further reduced.
In the aforementioned recording and reproduction method, as the information recording media, an information recording medium with a two-layer structure may be used, which includes: a first information recording surface corresponding to the light with the first wavelength; a second information recording surface corresponding to the light with the second wavelength; and first and second protective layers formed on the respective first and second information recording surfaces. Recording and/or reproduction may be carried out by projecting lights on each of the information recording surfaces from the same side, and the thickness of the first protective layer may be set so that chromatic aberration caused by the objective lens due to the chromatic dispersion caused by the material of the objective lens is reduced with respect to the light with the first wavelength. By varying the thickness of the protective layers in the information recording medium depending on the wavelengths of lights emitted from the light sources used, chromatic aberration caused by the objective lens can be reduced. Furthermore, by designing an information recording medium so as to have a two-layer structure, the recording capacity of the information recording medium itself can be increased.
In the aforementioned recording and reproduction method, as the information recording media, an information recording medium with a four-layer structure also may be used, which is obtained by bonding lamination units together by their information recording surface sides. Each of the lamination units is formed of a first information recording surface corresponding to the light with the first wavelength, a second information recording surface corresponding to the light with the second wavelength, and first and second protective layers formed on the respective first and second information recording surfaces. In the information recording medium, recording and/or reproduction can be carried out by projecting lights from both sides. By using the information recording medium for two-sided recording or reproduction that is obtained by bonding the information recording media with the two-layer structure by their information recording surface sides, the recording capacity of the information recording medium itself can be further increased.
In the aforementioned recording and reproduction method, it is preferable that the first wavelength is in a range between 350 nm and 500 nm and the second wavelength is in a range between 600 nm and 700 nm. According to this, using the same optical head, recording and/or reproduction of a plurality of optical disks with different specifications, such as high-density optical disks, DVDs, DVD-Rs, or the like, can be carried out.